Honestly if the rewards for the foundry quests weren't such ass I'd do nothing but those all day. Holy hell are some of them really fun and/or hilarious. If you haven't, find "Tired of being the hero" and do that one. It's cheesy as hell but I giggled at the end.
It's been over a year since I played but I'm
99% certain Cryptic keeps an "average time to complete" metric for every Foundry quest because the only ones eligible for the Daily quest have to exceed a specific minimum time to complete (think it's 15 minutes). The most popular speedrun foundry dungeons frequently have to be updated to add some extra rooms and mobs to get them back over the minimum time requirement.
Using that time metric, Cryptic could improve the completion reward by basing it on that metric. For example, less than 30 minutes you just get daily quest credit;
30-45 minutes you get daily credit plus some astral diamonds, 45-60 all the previous reward plus chance at a blue item, 60+ minutes all the previous awards with a chance for some significant loot reward.
It's been a while and I can't remember the name of it but there was one Foundry dungeon that stated up front it required a full group, mob difficulty was very high, and it took 1-2 hours to complete. I wanted to run it but guildmates who had already ran it had no interest in repeating it. I remember them saying it was one of the best Foundry dungeons available in terms of design, story, etc but there just wasn't any point to run it more than once due to the crappy reward.
Seems to me enhancing the reward incentive for the Foundry dungeons would be something Cryptic should be willing to do to increase the number of people playing their game. If the creator of a Foundry dungeon did a really good job, the game allows you to send them astral diamonds as a tip. So not only do you create an incentive for players to create quality content, you provide an incentive for players to run it. Seems like a win-win scenario to me.